1. Field of the Invention
The present invention relates to a process for refining coal-derived heavy carbonaceous materials such as pitch and the like, and more particularly to a process for refining coal-derived heavy carbonaceous materials so as to remove quinoline insolubles therefrom, thereby making the carbonaceous materials suitable for use in the production of so-called "high-grade carbon stocks" such as needle coke (which is readily graphitizable and is useful in making ultra high power graphitic electrodes), carbon fibers, high-purity carbon, etc., and for use as an impregnant or binder pitch.
2. Prior Art
In the past, various carbon stocks such as the above-mentioned high-grade carbon stocks, as well as activated carbon, carbon black, and isotropic carbon have been produced from a wide variety of raw materials including petroleum-derived heavy oils, coal-derived heavy oils, synthetic polymers, and wood cellulose. Of these, petroleum-derived heavy oils and coal-derived heavy oils have been used in much greater quantities because they are less expensive and give higher carbonization yields. In the production of carbon stocks, these heavy oils are typically utilized in the form of pitches such as tar pitches or petroleum pitches from which most of the distillable components have been removed.
Compared with petroleum-derived heavy oils, coal-derived heavy oils are advantageous in that they have a very low sulfur content and hence can achieve even higher carbonization yields. It is known, however, that coal-derived heavy oils contain slight amounts of quinoline insolubles which are composed predominantly of inactive or non-graphitizable carbonaceous matter and ashes, and that the presence of quinoline insolubles even in slight amounts has detrimental effects on the production of high-grade carbon stocks such as needle coke, carbon fibers, and the like.
For example, in the production of carbon fibers from tar pitch, quinoline insolubles present in the pitch may cause breaking of the extruded filament or blockage of the extrusion nozzle during melt spinning operation. When a coal-derived heavy oil is used as an impregnant, quinoline insolubles may cause blocking of pores of the porous body to be impregnated therewith, thereby inhibiting further impregnation or slowing down the impregnation speed. Furthermore, since quinoline insolubles exhibit inhibitory effects on graphitization of tar pitches, readily graphitizable needle coke cannot be produced from those tar pitches containing quinoline insolubles.
Thus, there is a need for a method of refining coal-derived heavy oils which can remove substantially all quinoline insolubles present therein in order to facilitate extensive use thereof in the production of high-grade carbon stocks, thereby decreasing the cost of such carbon stocks.
Many methods for removing quinoline insolubles from coal-derived heavy oils have been proposed in the art. Most of these methods involve heat treatment or solvent treatment of the heavy oils. However, these prior art refining methods for coal-derived heavy oils are generally unsatisfactory when they are performed on a commercial scale.
A commercially feasible refining process of coal-derived heavy oils is disclosed in Sunami et al. U.S. Pat. No. 4,402,824 (counterpart of West German Patent 3,112,004) which is incorporated herein by reference. The process comprises mixing a coal-derived heavy oil such as coal tar which has been freed of lighter components having boiling points up to at least 200.degree. C. and not more than 270.degree. C. with a ketone solvent such as acetone having a boiling point below 200.degree. C. to precipitate quinoline insolubles as coarse particles, which are then easily removed from the supernatant liquid. The ketone solvent is then readily recovered from the heavy oil by distillation for recycling to the mixing step because of the relatively low boiling point of the solvent.
Unfortunately, the process of Sunami et al. U.S. Pat. No. 4,402,824 cannot be applied to the refining of heavier oils or heavier carbonaceous materials such as tar pitches from which at least some of the distillable components having boiling points above 270.degree. C. such as anthracene oil have been removed in addition to those having boiling points of up to 270.degree. C., since it is difficult to uniformly mix such heavier materials with the ketone solvent for refining. More specifically, since such heavier materials are normally solid at room temperature, it takes a long period to mix them with the ketone solvent substantially completely. In addition, during the mixing, sticky gum substances are often formed, and these stick to the walls of the mixing equipment or piping, thereby interfering with smooth and continuous mixing. Furthermore, the gum substances cannot be readily removed by conventional separating techniques such as sedimentation, centrifuging, or filtration.
Thus, the process of Sunami et al. is essentially limited to the refining of coal-derived heavy oils from which only lighter components having boiling points of at most 270.degree. C. have been removed. In order to use such a refined heavy oil in the production of carbon stocks by a process which involves carbonization, it must usually be converted to a much heavier material such as a pitch prior to the carbonization step by removing at least some of the distillable components boiling above 270.degree. C. (the so-called "middle and heavy distillates") of the oil by any suitable means such as distillation or polymerization into higher molecular weights. The distillation or polymerization, however, often causes re-formation of undesirable quinoline insolubles, and in such cases the resulting heavier material such as pitch must be refined again so as to be free from quinoline insolubles.
Therefore, it is highly desirable to refine a coal-derived heavy oil to remove quinoline insolubles after it has been freed of at least some of the distillable components boiling above 270.degree. C. such that the refined material can be directly used in the production of high-grade carbon stocks without distillation or polymerization, and further refining.
Accordingly, it is an object of the invention to provide a process for refining coal-derived heavy carbonaceous materials which is commercially feasible and which can be successfully applied to those coal-derived heavy carbonaceous materials from which all the components boiling at 270.degree. C. or below and at least some of the components boiling above 270.degree. C. have been removed.
It is another object of the invention to provide a process for refining such carbonaceous materials by means of solvent treatment to remove quinoline insolubles as readily-separable coarse precipitates.
It is a further object of the invention to provide a process for refining such carbonaceous materials so as to yield a refined material suitable for use in the production of high-grade, high-purity carbon stocks.
It is a still further object of the invention to provide a process for refining such carbonaceous materials by means of solvent treatment using a solvent capable of being readily mixed with the carbonaceous materials and of being readily removed therefrom.